Metal-containing oxidized cellulose nanofiber dispersion and method of producing the same

ABSTRACT

Disclosed is a dispersion of metal-containing oxidized cellulose nanofibers with superior dispersibility, which is applicable to various uses. The disclosed metal-containing oxidized cellulose nanofiber dispersion comprises a dispersion medium, and metal-containing oxidized cellulose nanofibers containing a metal other than sodium in salt form, wherein the metal-containing oxidized cellulose nanofibers have a number-average fiber diameter of 100 nm or less.

TECHNICAL FIELD

The present disclosure relates to metal-containing oxidized cellulosenanofiber dispersions and methods of producing the same.

BACKGROUND

Methods of producing a dispersion have heretofore been proposed whereinnative cellulose is oxidized in the presence of an oxidizing catalystsuch as N-oxyl compound and the resulting oxidized cellulose issubjected to mechanical dispersing treatment to prepare a dispersion inwhich highly crystalline ultrafine fibers (oxidized cellulosenanofibers) with several nanometer diameters are dispersed in adispersion medium such as water (see, e.g., PTL 1). Oxidized cellulosenanofiber dispersions produced by such production methods compriseindividual strands of oxidized cellulose nanofiber separated from oneanother in the dispersion medium and are expected to be applied tovarious uses such as composite materials.

CITATION LIST Patent Literature

PTL 1: JP2008-1728A

SUMMARY Technical Problem

It is important to further improve the performance of oxidized cellulosenanofibers according to the intended application when they are appliedto various uses such as composite materials.

Thus, for practical uses of oxidized cellulose nanofibers, there hasbeen a need in the art to provide a technique of imparting desiredcharacteristics to the oxidized cellulose nanofibers while retainingtheir dispersibility.

Solution to Problem

The inventors made extensive studies to provide a technique of impartingdesired characteristics to oxidized cellulose nanofibers while retainingtheir dispersibility. The inventors focused on the fact that theabove-described conventional method of producing an oxidized cellulosenanofiber dispersion involves the use of sodium hypochlorite and sodiumbromide as a co-oxidizing agent upon oxidation of native cellulose inthe presence of an oxidizing catalyst, so that the primary hydroxylgroups at C6 of the β-glucose units (building blocks of cellulose) ofthe oxidized cellulose nanofibers in the resulting dispersion areoxidized to sodium carboxylate groups (sodium salts of carboxyl groups).Further, the inventors conceived a new idea of imparting desiredcharacteristics to oxidized cellulose nanofibers by substituting thesodium ion part of the sodium carboxylate of the oxidized cellulosenanofibers by an ion of a metal other than sodium to formmetal-containing oxidized cellulose nanofibers containing a metal otherthan sodium in salt form. The inventors then made studies andestablished that it is possible to provide a dispersion containingwell-dispersed metal-containing oxidized cellulose nanofibers containinga metal other than sodium in salt form, by contacting oxidized cellulosenanofibers dispersed in a dispersion medium with a salt containing ametal other than sodium. The inventors completed the present disclosurebased on these findings.

Specifically, the present disclosure is aimed at advantageously solvingthe foregoing problem, and the disclosed metal-containing oxidizedcellulose nanofiber dispersion comprises a dispersion medium, andmetal-containing oxidized cellulose nanofibers containing a metal otherthan sodium in salt form, wherein the metal-containing oxidizedcellulose nanofibers have a number-average fiber diameter of 100 nm orless.

In such a dispersion which comprises metal-containing oxidized cellulosenanofibers having a number-average fiber diameter of 100 nm or less andcontaining a metal other than sodium in salt form, the metal-containingoxidized cellulose nanofibers exhibit superior dispersibility, andappropriate selection of the type of the metal contained in salt formallows desired characteristics to be imparted to the metal-containingoxidized cellulose nanofibers. Thus, when the dispersion is used forexample to form composite material, it is possible to allow thecomposite material to show desired characteristics even when the blendedamount is small.

The present disclosure is also aimed at advantageously solving theforegoing problem, and a first disclosed method of producing ametal-containing oxidized cellulose nanofiber dispersion includescontacting oxidized cellulose nanofibers containing a first metal insalt form, dispersed in a solvent, with a salt of a second metal otherthan the first metal to provide metal-containing oxidized cellulosenanofibers containing the second metal in salt form, wherein themetal-containing oxidized cellulose nanofibers have a number-averagefiber diameter of 100 nm or less.

Contacting oxidized cellulose nanofibers containing a first metal insalt form, dispersed in a solvent, with a salt of a second metal asdescribed above allows for easy production of a dispersion ofmetal-containing oxidized cellulose nanofibers containing the secondmetal in salt form and having a number-average fiber diameter of 100 nmor less.

The present disclosure is also aimed at advantageously solving theforegoing problem, and a second disclosed method of producing ametal-containing oxidized cellulose nanofiber dispersion includescontacting oxidized cellulose nanofibers containing a first metal insalt form, dispersed in a solvent, with a strong acid to substitute ionsof the first metal contained in salt form by hydrogen atoms; andcontacting the oxidized cellulose nanofibers in which the ions of thefirst metal have been substituted by hydrogen atoms, dispersed in asolvent, with a salt of a second metal other than the first metal toprovide metal-containing oxidized cellulose nanofibers containing thesecond metal in salt form, wherein the metal-containing oxidizedcellulose nanofibers have a number-average fiber diameter of 100 nm orless.

Contacting oxidized cellulose nanofibers in which ions of a first metalhave been substituted by hydrogen atoms, dispersed in a solvent, with asalt of a second metal as described above can efficiently promote thesubstitution reaction of the first metal and therefore allows forefficient production of a dispersion of metal-containing oxidizedcellulose nanofibers containing the second metal in salt form and havinga number-average fiber diameter of 100 nm or less.

In the present disclosure, the “number-average fiber diameter” ofmetal-containing oxidized cellulose nanofibers can be found by measuringthe fiber diameters of 5 or more metal-containing oxidized cellulosenanofibers using an atomic force microscope, and calculating the numberaverage of measured fiber diameters. Specifically, the “number-averagefiber diameter” of metal-containing oxidized cellulose nanofibers can befound for example using the measurement method described in Examplesdisclosed herein.

In the present disclosure, the oxidized cellulose nanofibers arepreferably carboxylated cellulose nanofibers, and the metal-containingoxidized cellulose nanofibers are preferably metal-containingcarboxylated cellulose nanofibers.

Metal-containing carboxylated cellulose nanofibers produced usingcarboxylated cellulose nanofibers exhibit superior dispersibility andsufficiently allow composite material and the like to show desiredcharacteristics even when the blended amount is small.

In the present disclosure, the metal-containing oxidized cellulosenanofibers preferably have a number-average fiber length of 50 nm to2,000 nm. When the number-average fiber length of the metal-containingoxidized cellulose nanofibers is 50 nm to 2,000 nm, it is possible toimpart sufficiently high mechanical strength to composite material andthe like while ensuring dispersibility.

In the present disclosure, the “number-average fiber length” of themetal-containing oxidized cellulose nanofibers can be found by measuringthe fiber lengths of 5 or more metal-containing oxidized cellulosenanofibers using an atomic force microscope, and calculating the numberaverage of measured fiber lengths. Specifically, the “number-averagefiber length” of metal-containing oxidized cellulose nanofibers can befound for example using the measurement method described in Examplesdisclosed herein.

In the present disclosure, the metal-containing oxidized cellulosenanofibers preferably have an average degree of polymerization of 100 to2,000. When the average degree of polymerization of the metal-containingoxidized cellulose nanofibers is 100 to 2,000, it is possible to impartsufficiently high mechanical strength to composite material and the likewhile ensuring dispersibility.

In the present disclosure, the “average degree of polymerization” of themetal-containing oxidized cellulose nanofibers can be found by theviscosity method.

In the disclosed metal-containing oxidized cellulose nanofiberdispersion, the metal other than sodium is preferably at least one metalselected from the group consisting of metals of Group 2 to Group 14 inPeriod 3 to Period 6 of the long periodic table, more preferably atleast one metal selected from the group consisting of magnesium,aluminum, calcium, titanium, chromium, manganese, iron, cobalt, nickel,copper, zinc, silver, tin, barium, and lead, even more preferably atleast one metal selected from the group consisting of aluminum, calcium,iron, cobalt, copper, zinc, and silver.

In the disclosed method of producing a dispersion, it is preferred thatthe first metal is sodium and the second metal is at least one metalselected from the group consisting of metals of Group 2 to Group 14 inPeriod 3 to Period 6 of the long periodic table, more preferably atleast one metal selected from the group consisting of magnesium,aluminum, calcium, titanium, chromium, manganese, iron, cobalt, nickel,copper, zinc, silver, tin, barium, and lead, even more preferably atleast one metal selected from the group consisting of aluminum, calcium,iron, cobalt, copper, zinc, and silver.

The use of these metals allows desired characteristics to be imparted tothe metal-containing oxidized cellulose nanofibers.

In the disclosed metal-containing oxidized cellulose nanofiberdispersion, the dispersion medium is preferably water.

Also in the disclosed method of producing a dispersion, the solvent ispreferably water.

The use of water as the dispersion medium or solvent allows themetal-containing oxidized cellulose nanofibers to be well dispersed inthe dispersion.

Advantageous Effect

According to the present disclosure, it is possible to provide adispersion of metal-containing oxidized cellulose nanofibers withsuperior dispersibility, which is applicable to various uses.

DETAILED DESCRIPTION

The present disclosure will be described in detail below.

The disclosed method of producing a metal-containing oxidized cellulosenanofiber dispersion can be used for example to produce the disclosedmetal-containing oxidized cellulose nanofiber dispersion. Themetal-containing oxidized cellulose nanofiber dispersion produced usingthe disclosed method of producing a metal-containing oxidized cellulosenanofiber dispersion is suitably used in various applications, includingformation of composite materials. The following describes the disclosedmethod of producing a metal-containing oxidized cellulose nanofiberdispersion and the disclosed metal-containing oxidized cellulosenanofiber dispersion which may be produced using the method.

(Method of Producing Metal-Containing Oxidized Cellulose NanofiberDispersion)

Using oxidized cellulose nanofibers containing a first metal in saltform as a raw material, the disclosed production method uses either oneof the following methods (i) and (ii) to substitute ions of the firstmetal of the oxidized cellulose nanofibers by ions of a second metal toproduce a dispersion of metal-containing oxidized cellulose nanofiberscontaining the second metal in salt form and having a number-averagefiber diameter of 100 nm or less:

Method (i): oxidized cellulose nanofibers containing a first metal insalt form, dispersed in a solvent, are contacted with a salt of a secondmetal other than the first metal (first production method); and

Method (ii): oxidized cellulose nanofibers containing a first metal insalt form, dispersed in a solvent, are contacted with a strong acid tosubstitute ions of the first metal contained in salt form by hydrogenatoms, after which the oxidized cellulose nanofibers in which the ionsof the first metal have been substituted by hydrogen atoms, dispersed ina solvent, are contacted with a salt of a second metal other than thefirst metal (second production method).

<First Production Method>

In the first production method, oxidized cellulose nanofibers containinga first metal in salt form, dispersed in a solvent, are contacted with asalt of a second metal other than the first metal to substitute at leastsome, preferably all, of the ions of the first metal of the oxidizedcellulose nanofibers by ions of the second metal (metal substitutionstep). The metal-containing oxidized cellulose nanofibers containing thesecond metal in salt form obtained from the metal substitution step arethen optionally washed (washing step), and further dispersed in adispersion medium (dispersing step) to afford a dispersion ofmetal-containing oxidized cellulose nanofibers dispersed in thedispersion medium, the metal-containing oxidized cellulose nanofiberscontaining the second metal in salt form and having a number-averagefiber diameter of 100 nm or less.

[Metal Substitution Step]

For the oxidized cellulose nanofibers containing a first metal in saltform which may be used in the metal substitution step, any oxidizedcellulose nanofibers can be used as long as they are obtainable byoxidation of cellulose and contain the first metal in salt form, e.g.,oxidized cellulose nanofibers disclosed in WO2011/074301 can be used. Inparticular, it is preferred to use carboxylated cellulose nanofiberscontaining the first metal in salt form. The use of carboxylatedcellulose nanofibers results in a dispersion of metal-containingoxidized cellulose nanofibers having superior dispersibility.

Any carboxylated cellulose nanofibers containing the first metal in saltform can be used. For example, carboxylated cellulose nanofibers inwhich the primary hydroxyl groups at C6 of the β-glucose units (buildingblocks of cellulose) are selectively oxidized can be used. Examples ofmethods of selectively oxidizing the primary hydroxyl groups at C6 ofthe β-glucose units include oxidation methods that use N-oxyl compoundsas an oxidation catalyst, such as TEMPO-catalyzed oxidation describedbelow.

In TEMPO-catalyzed oxidation, native cellulose as a raw material isoxidized in an aqueous medium by the action of an oxidizing agent usingTEMPO (2,2,6,6-tetramethyl-1-piperidine-N-oxyl) or a derivative thereofas an oxidation catalyst. The native cellulose subjected to theoxidation treatment is then optionally washed and then dispersed in anaqueous medium such as water to afford an aqueous dispersion ofcellulose nanofibers having a number-average fiber diameter of, forexample, 100 nm or less, preferably 10 nm or less, and having a group inthe form of carboxylate (carboxylated cellulose nanofibers).

Native cellulose usable as the raw material can be purified celluloseisolated from cellulose biosynthesis system, such as a plant, animal orbacteria-producing gel. Specific examples include cellulose isolatedfrom coniferous wood pulp, deciduous wood pulp, cotton-based pulp suchas cotton linter or cotton lint, non-wood-based pulp such as pulp frombarley or bagasse pulp, bacterial cellulose, cellulose isolated from seasquirt, and cellulose isolated from sea grass.

From the perspective of increasing the efficiency of the oxidationreaction and thus the productivity of carboxylated cellulose nanofibers,isolated, purified native cellulose may be subjected to beating or othertreatment to increase the surface area. Further, it is preferred to usenever-dried native cellulose that has been stored in a never-dried stateafter isolation and purification, since by doing so, bundles ofmicrofibrils are kept in a state that allows for easy swelling, therebyimproving the oxidation reaction efficiency and facilitating theproduction of carboxylated cellulose nanofibers with small fiberdiameters.

TEMPO derivatives usable as oxidation catalysts include those havingvarious functional groups at C4 of2,2,6,6-tetramethyl-1-piperidine-N-oxyl (TEMPO). Examples of TEMPOderivatives include 4-acetamido-TEMPO, 4-carboxy-TEMPO, and4-phosphonoxy-TEMPO. High reaction rate can be attained especially whenTEMPO or 4-acetamido-TEMPO is used as an oxidation catalyst.

Examples of oxidizing agents include hypohalous acids or salts thereof(e.g., hypochlorous acid or salt thereof, hypobromous acid or saltthereof, and hypoiodous acid or salt thereof); halous acids or saltsthereof (e.g., chlorous acid or salt thereof, bromous acid or saltthereof, and iodous acid or salt thereof); perhalogen acids or saltsthereof (e.g., perchloric acid or salt thereof, and periodic acid orsalt thereof); halogens (e.g., chlorine, bromine, and iodine); halogenoxides (e.g., ClO, ClO₂, Cl₂O₆, BrO₂, and Br₃O₇); nitrogen oxides (e.g.,NO, NO₂, and N₂O₃); and peracids (e.g., hydrogen peroxide, peraceticacid, persulfuric acid, and perbenzoic acid). These oxidizing agents canbe used alone or in combination, and also can be used in combinationwith oxidizing enzymes such as laccase.

Depending on the type of the oxidizing agent, bromide or iodide may becombined with the oxidizing agent for use as a co-oxidizing agent. Forexample, ammonium salts (ammonium bromide, ammonium iodide), alkalimetal bromides or iodides, and alkaline earth metal bromides or iodidescan be used. These bromides and iodides can be used alone or incombination.

When a metal salt has been used as the oxidizing agent duringTEMPO-catalyzed oxidation, generally, the metal constituting the metalsalt is contained in salt form in the resulting carboxylated cellulosenanofibers. That is, the metal constituting the metal salt is the firstmetal.

Among the oxidizing agents described above, it is preferred to use asodium salt from the perspective of increasing the oxidation reactionrate, with sodium hypochlorite being more preferred, and a co-oxidizingagent of sodium hypochlorite and sodium bromide being particularlypreferred. When a sodium salt has been used as the oxidizing agent,generally, carboxylated cellulose nanofibers containing sodium in saltform as the first metal are obtained.

Any condition and method known in the art used for TEMPO-catalyzedoxidation can be employed for the oxidation treatment. In the oxidationtreatment, the primary hydroxyl groups at C6 of the β-glucose units areoxidized via aldehyde groups to carboxyl groups. From the perspective ofimparting sufficient levels of desired characteristics tometal-containing oxidized cellulose nanofibers obtained from the rawmaterial carboxylated cellulose nanofibers, the proportion of theprimary hydroxyl groups oxidized to carboxyl groups is preferably 50 mol% or more, more preferably 70 mol % or more, even more preferably 90 mol% or more.

Various dispersing devices (defibration devices) can be used fordispersing the carboxylated cellulose nanofibers after the oxidationtreatment. Specifically, for example, a defibration device such as ahousehold mixer, an ultrasonic homogenizer, a high-pressure homogenizer,a twin-screw kneader or a stone mill can be used. In addition,defibration devices that are commonly used for domestic use orindustrial production can be used. In particular, defibration deviceswith a stronger beating power, such as various homogenizers andrefiners, more efficiently provide a dispersion of carboxylatedcellulose nanofibers with small fiber diameters.

It is preferred that the carboxylated cellulose nanofibers after theoxidation treatment are dispersed after increasing the purity byrepeated cycles of washing with water and solid-liquid separation. Inaddition, when non-defibrated components remain in the dispersion afterdispersing treatment, it is preferred to remove such non-defibratedcomponents by centrifugation or other techniques.

In the metal substitution step, substitution of metal ions by a contactbetween oxidized cellulose nanofibers containing a first metal in saltform and a salt of a second metal can be accomplished by adding asolution or solid of the salt of the second metal to a dispersion ofoxidized cellulose nanofibers obtained through the above-describedTEMPO-catalyzed oxidation or other oxidation methods, and stirring theresulting mixture.

The salt of the second metal can be a salt of a metal which conforms tocharacteristics desired to be imparted to the resulting metal-containingoxidized cellulose nanofibers. Specifically, the salt of the secondmetal is, for example when the first metal is sodium (i.e., when asodium salt is used as the oxidizing agent), not particularly limited,and preferably can be a salt of at least one metal selected from themetals of Group 2 to Group 14 in Period 3 to Period 6 of the longperiodic table, more preferably can be a salt of at least one metalselected from the group consisting of magnesium, aluminum, calcium,titanium, chromium, manganese, iron, cobalt, nickel, copper, zinc,silver, tin, barium, and lead, even more preferably can be a salt of atleast one metal selected from the group consisting of aluminum, calcium,iron, cobalt, copper, zinc, and silver.

The second metal to be added to the dispersion of oxidized cellulosenanofibers can be in any salt form, such as halide, acetate, sulfate, ornitrate. In particular, from the perspective of improving the efficiencywith which metal ions are substituted, the salt of the second metal ispreferably a weak acid salt, more preferably an acetate.

Further, from the perspective of effective metal substitution onwell-dispersed oxidized cellulose nanofibers, the dispersion of theoxidized cellulose nanofibers containing the first metal in salt form ispreferably an aqueous dispersion. The concentration of the oxidizedcellulose nanofibers in the dispersion is preferably 0.005% by mass ormore, more preferably 0.01% by mass or more, even more preferably 0.05%by mass or more, and preferably 5% by mass or less, more preferably 3%by mass or less, even more preferably 2% by mass or less. If theconcentration of the oxidized cellulose nanofibers is too low, itresults in poor reaction efficiency and productivity. If theconcentration of the oxidized cellulose nanofibers is too high, itresults in high dispersion viscosity making uniform stirring difficultto perform.

The time of stirring the mixture of the oxidized cellulose nanofibersand the salt of the second metal can be a time sufficient for effectingmetal ion substitution, e.g., from 1 hour to 10 hours. Stirringtemperature can for example range from 10° C. to 50° C.

In the metal substitution step described above, when the oxidizedcellulose nanofibers containing the first metal in salt form arecontacted with the salt of the second metal in liquid, gelling of theoxidized cellulose nanofibers may occur. Even in such a case, performingthe dispersing step after the optional washing step allows the resultingoxidized cellulose nanofibers to be well re-dispersed, so that adispersion of metal-containing oxidized cellulose nanofibers having anumber-average fiber diameter of 100 nm or less can be obtained.

[Washing Step]

In the optional washing step that follows the metal substitution step,the oxidized cellulose nanofibers after metal substitution are washed byany washing method known in the art, e.g., repeated cycles ofcentrifugation and replacement of supernatant with washing solution, orfiltration and washing with a large quantity of washing solution.

Any washing solution can be used, e.g., water can be used. However, fromthe perspective of enhancing the efficiency of metal substitution of theresulting oxidized cellulose nanofiber obtained from the metalsubstitution step, it is preferred to perform washing first using anaqueous solution of the salt of the second metal as washing solution,and then using water as washing solution.

[Dispersing Step]

In the dispersing step, the oxidized cellulose nanofibers containing thesecond metal in salt form are dispersed using a known dispersing device(defibration device) such as a household mixer, an ultrasonichomogenizer, a high-pressure homogenizer, a twin-screw kneader, or astone mill. Non-defibrated components are removed where necessary bycentrifugation or other techniques to provide a dispersion ofmetal-containing oxidized cellulose nanofibers containing the secondmetal in salt form.

In the dispersion obtained as described above, the metal-containingoxidized cellulose nanofibers containing the second metal in salt formare highly dispersed to an extent that the metal-containing oxidizedcellulose nanofibers have a number-average fiber diameter of 100 nm orless, preferably 2 nm to 10 nm, more preferably 2 nm to 5 nm. Thus, withthe dispersion, it is possible to impart desired characteristics tocomposite material or the like in a good manner even when the amountused is small.

The metal-containing oxidized cellulose nanofibers containing the secondmetal in salt form obtained in the manner described above preferablyhave a number-average fiber length of 50 nm to 2,000 nm, more preferably70 nm to 1,500 nm, even more preferably 100 nm to 1,000 nm, particularlypreferably 400 nm to 600 nm. When the number-average fiber length is 50nm or more, it is possible to sufficiently increase the mechanicalstrength of the metal-containing oxidized cellulose nanofibers andcomposite material containing the metal-containing oxidized cellulosenanofibers and therefore to impart sufficiently high mechanical strengthto a shaped article formed using an aggregate or composite material ofthe metal-containing oxidized cellulose nanofibers. When thenumber-average fiber length is 2,000 nm or less, the dispersibility ofthe metal-containing oxidized cellulose nanofibers can be ensured, sothat the dispersion can be sufficiently enriched with themetal-containing oxidized cellulose nanofibers.

The number-average fiber length of the metal-containing oxidizedcellulose nanofibers containing the second metal in salt form can beadjusted for example by changing the number-average fiber length of theraw material native cellulose and the oxidizing treatment conditions,the condition used for dispersing (defibrating) the carboxylatedcellulose nanofibers after the oxidation treatment, and/or the conditionused for dispersing (defibrating) the oxidized cellulose nanofiberscontaining the second metal in salt form after the metal substitutionstep. Specifically, by prolonging the time of dispersing treatment(defibrating treatment) or by increasing the energy applied duringdispersing treatment (defibrating treatment), the number-average fiberlength can be reduced.

The metal-containing oxidized cellulose nanofibers containing the secondmetal in salt form preferably have an average degree of polymerization(average number of glucose units in the cellulose molecules) of 100 to2,000, more preferably 300 to 1,500, even more preferably 500 to 1,000,particularly preferably 500 to 700. When the average degree ofpolymerization is 100 or more, it is possible to sufficiently increasethe mechanical strength of the metal-containing oxidized cellulosenanofibers and composite material containing the metal-containingoxidized cellulose nanofibers and therefore to impart sufficiently highmechanical strength to a shaped article formed using an aggregate orcomposite material of the metal-containing oxidized cellulosenanofibers. When the average degree of polymerization is 2,000 or less,the dispersibility of the metal-containing oxidized cellulose nanofiberscan be ensured, so that the dispersion can be sufficiently enriched withthe metal-containing oxidized cellulose nanofibers.

The average degree of polymerization of the metal-containing oxidizedcellulose nanofibers containing the second metal in salt form can beadjusted for example by changing the average degree of polymerization ofthe raw material native cellulose and the oxidizing treatmentconditions, the condition used for dispersing (defibrating) thecarboxylated cellulose nanofibers after the oxidation treatment, and/orthe condition used for dispersing (defibrating) the oxidized cellulosenanofibers containing the second metal in salt form after the metalsubstitution step.

<Second Production Method>

In the second production method, first, oxidized cellulose nanofiberscontaining a first metal in salt form, dispersed in a solvent, arecontacted with a strong acid to substitute at least some, preferablyall, of the ions of the first metal of the oxidized cellulose nanofibersby hydrogen atoms (hydrogen substitution step). Next, the oxidizedcellulose nanofibers obtained from the above hydrogen substitution stepare optionally washed (first washing step) and further dispersed in adispersion medium (first dispersing step). Subsequently, the oxidizedcellulose nanofibers in which the ions of the first metal have beensubstituted by hydrogen atoms, dispersed in a solvent, are contactedwith a salt of a second metal to substitute at least some, preferablyall of, the hydrogen atoms introduced in the hydrogen substitution stepand the ions of the first metal which have not been substituted byhydrogen atoms (metal substitution step). Thereafter, themetal-containing oxidized cellulose nanofibers containing the secondmetal in salt form, obtained from the metal substitution step, areoptionally washed (second washing step), and further dispersed in adispersion medium (second dispersing step) to afford a dispersion ofmetal-containing oxidized cellulose nanofibers dispersed in thedispersion medium, the metal-containing oxidized cellulose nanofiberscontaining the second metal in salt form and having a number-averagefiber diameter of 100 nm or less.

In this second production method, since the hydrogen substitution stepprecedes the metal substitution step, it is possible to increase thesubstitution of the first metal by the second metal compared to thefirst production method described above where the first metal isdirectly substituted by the second metal.

[Hydrogen Substitution Step]

The oxidized cellulose nanofibers containing the first metal in saltform in the hydrogen substitution step can be the oxidized cellulosenanofibers used in the above-described first production method.

In the hydrogen substitution step, substitution of ions of the firstmetal by hydrogen atoms by a contact between the oxidized cellulosenanofibers containing the first metal in salt form and a strong acid canbe accomplished by adding a solution of a strong acid to a dispersion ofoxidized cellulose nanofibers obtained by TEMPO-catalyzed oxidation orother oxidation methods, and stirring the resulting mixture.

Any strong acid can be used as long as it is capable of substitutingions of the first metal by hydrogen atoms (i.e., substituting thecarboxyl groups of the oxidized cellulose nanofibers by carboxylic acidform). For example, it is possible to use hydrochloric acid, sulfuricacid or nitric acid, with hydrochloric acid being preferred.

The time of stirring the mixture of the oxidized cellulose nanofibersand strong acid can be a time sufficient for effecting substitution ofmetal ions by hydrogen atoms, e.g., from 10 minutes to 5 hours. Stirringtemperature can for example range from 10° C. to 50° C.

[First Washing Step]

In the optional first washing step that follows the hydrogensubstitution step, the hydrogen-substituted oxidized cellulosenanofibers are washed to remove strong acid by any washing method knownin the art, e.g., repeated cycles of centrifugation and replacement ofsupernatant with washing solution, or filtration and washing with alarge quantity of washing solution. In this way, by carrying out thefirst washing step, it is possible to remove strong acid and to preventcarboxyl groups of carboxylic acid form from remaining in the metalsubstitution step described later. As a result, in the metalsubstitution step, the hydrogen atoms introduced in the hydrogensubstitution step and the ions of the first metal which have not beensubstituted by hydrogen atoms can be sufficiently substituted by ions ofthe second metal.

Any washing solution can be used in the first washing step, e.g., watercan be used. However, from the perspective of enhancing the efficiencywith which the carboxyl groups of the oxidized cellulose nanofibers aresubstituted by carboxylic acid form, it is preferred to perform washingfirst using a solution of strong acid as washing solution, and thenusing water as washing solution.

[First Dispersing Step]

In the first dispersing step, the oxidized cellulose nanofibers in whichcarboxyl groups are substituted by carboxylic acid form are dispersed ina dispersion medium such as water to afford a dispersion of oxidizedcellulose nanofibers in which ions of the first metal are substituted byhydrogen atoms. In the first dispersing step, the oxidized cellulosenanofibers in which carboxyl groups are substituted by carboxylic acidform need not be completely dispersed in the dispersion medium using aknown dispersing device (defibrating device) or the like.

[Metal Substitution Step]

The metal substitution step of the second production method can beperformed in the same way as that of the first production method exceptthat oxidized cellulose nanofibers in which ions of the first metal aresubstituted by hydrogen atoms are contacted with a salt of a secondmetal. A preferred mode of the metal substitution step of the secondproduction method is also the same as that of the metal substitutionstep of the first production method.

[Second Washing Step and Second Dispersing Step]

The second washing step and the second dispersing step in the secondproduction method can also be performed in the same way as those of thefirst production method described above. Further, preferred modes of thesecond washing step and the second dispersing step of the secondproduction method are also the same as those of the washing step and thedispersing step of the first production method.

In the dispersion obtained as described above, the metal-containingoxidized cellulose nanofibers containing the second metal in salt formare highly dispersed to an extent that the metal-containing oxidizedcellulose nanofibers have a number-average fiber diameter of 100 nm orless, preferably 2 nm to 10 nm, more preferably 2 nm to 5 nm. Thus, withthe dispersion, it is possible to impart desired characteristics tocomposite material or the like in a good manner even when the amountused is small.

The metal-containing oxidized cellulose nanofibers containing the secondmetal in salt form obtained as described above preferably have anumber-average fiber length of 50 nm to 2,000 nm, more preferably 70 nmto 1,500 nm, even more preferably 100 nm to 1,000 nm, particularlypreferably 400 nm to 600 nm. When the number-average fiber length is 50nm or more, it is possible to sufficiently increase the mechanicalstrength of the metal-containing oxidized cellulose nanofibers andcomposite material containing the metal-containing oxidized cellulosenanofibers and therefore to impart sufficiently high mechanical strengthto a shaped article formed using an aggregate or composite material ofthe metal-containing oxidized cellulose nanofibers. When thenumber-average fiber length is 2,000 nm or less, the dispersibility ofthe metal-containing oxidized cellulose nanofibers can be ensured, sothat the dispersion can be sufficiently enriched with themetal-containing oxidized cellulose nanofibers.

The number-average fiber length of the metal-containing oxidizedcellulose nanofibers containing the second metal in salt form can beadjusted for example by changing the number-average fiber length of theraw material native cellulose and the oxidizing treatment conditions,the condition used for dispersing (defibrating) the carboxylatedcellulose nanofibers after the oxidation treatment, and/or the conditionused for dispersing (defibrating) the oxidized cellulose nanofiberscontaining the second metal in salt form after the metal substitutionstep. Specifically, by prolonging the time of dispersing treatment(defibrating treatment) or by increasing the energy applied duringdispersing treatment (defibrating treatment), the number-average fiberlength can be reduced.

The metal-containing oxidized cellulose nanofibers containing the secondmetal in salt form preferably have an average degree of polymerization(average number of glucose units in the cellulose molecules) of 100 to2,000, more preferably 300 to 1,500, even more preferably 500 to 1,000,and particularly preferably 500 to 700. When the average degree ofpolymerization is 100 or more, it is possible to sufficiently increasethe mechanical strength of the metal-containing oxidized cellulosenanofibers and composite material containing the metal-containingoxidized cellulose nanofibers and therefore to impart sufficiently highmechanical strength to a shaped article formed using an aggregate orcomposite material of the metal-containing oxidized cellulosenanofibers. When the average degree of polymerization is 2,000 or less,the dispersibility of the metal-containing oxidized cellulose nanofiberscan be ensured, so that the dispersion can be sufficiently enriched withthe metal-containing oxidized cellulose nanofibers.

The average degree of polymerization of the metal-containing oxidizedcellulose nanofibers containing the second metal in salt form can beadjusted for example by changing the average degree of polymerization ofthe raw material native cellulose and the oxidizing treatmentconditions, the condition used for dispersing (defibrating) thecarboxylated cellulose nanofibers after the oxidation treatment, and/orthe condition used for dispersing (defibrating) the oxidized cellulosenanofibers containing the second metal in salt form after the metalsubstitution step.

(Metal-Containing Oxidized Cellulose Nanofiber Dispersion)

The metal-containing oxidized cellulose nanofiber dispersion produced bythe above-described production method comprises, for example, adispersion medium such as water, and metal-containing oxidized cellulosenanofibers containing a metal other than sodium in salt form. In thedispersion, the metal-containing oxidized cellulose nanofibers arehighly dispersed to an extent that the metal-containing oxidizedcellulose nanofibers have a number-average fiber diameter of 100 nm orless, preferably 2 nm to 10 nm, more preferably 2 nm to 5 nm.

The metal other than sodium can be any metal and can be at least onemetal selected from the group consisting of the metals of Group 2 toGroup 14 in Period 3 to Period 6 of the long periodic table, preferablyat least one metal selected from the group consisting of magnesium,aluminum, calcium, titanium, chromium, manganese, iron, cobalt, nickel,copper, zinc, silver, tin, barium, and lead, more preferably at leastone metal selected from the group consisting of aluminum, calcium, iron,cobalt, copper, zinc, and silver.

As described above, the metal-containing oxidized cellulose nanofibersin the dispersion preferably have a number-average fiber length of 50 nmto 2,000 nm, more preferably 70 nm to 1,500 nm, even more preferably 100nm to 1,000 nm, particularly preferably 400 nm to 600 nm.

The metal-containing oxidized cellulose nanofibers in the dispersionpreferably have an average degree of polymerization of 100 to 2,000,more preferably 300 to 1,500, even more preferably 500 to 1,000,particularly preferably 500 to 700.

The metal-containing oxidized cellulose nanofiber dispersion can forexample be directly dried when forming a functional membrane formed ofmetal-containing oxidized cellulose nanofibers (aggregate ofmetal-containing oxidized cellulose nanofibers). Alternatively, themetal-containing oxidized cellulose nanofiber dispersion can be firstmixed with polymers or other materials to form composite materials whichare used to manufacture various shaped articles. Further, themetal-containing oxidized cellulose nanofiber dispersion can be usedwhen attaching metal-containing oxidized cellulose nanofibers to papersheets, fibers, shaped articles or the like by coating, spraying,impregnating or other techniques while retaining the dispersibility ofthe metal-containing oxidized cellulose nanofibers. Such functionalmembranes and shaped articles formed using the metal-containing oxidizedcellulose nanofiber dispersion, and such paper sheets, fibers and shapedarticles having the metal-containing oxidized cellulose nanofibersattached may exhibit performance according to the type of the metalcontained in the metal-containing oxidized cellulose nanofibers.

EXAMPLES

The following provides a more specific description of the presentdisclosure based on Examples, which however shall not be construed aslimiting. In the following description, “%” and “part(s)” used toexpress quantities are by mass, unless otherwise specified.

In Examples, the carboxyl group amount of oxidized cellulose nanofibers,and the number-average fiber diameter, number-average fiber length,degree of polymerization and metal amount of metal-containing oxidizedcellulose nanofibers were evaluated using the methods described below.

<Carboxyl Group Amount>

60 mL of a dispersion containing 0.5% to 1% by mass of oxidizedcellulose nanofibers was prepared from a pulp sample of oxidizedcellulose nanofibers precisely weighed in dry weight. Next, after the pHof the dispersion was adjusted to about 2.5 with 0.1M hydrochloric acid,changes in electrical conductivity were observed until the pH reached 11by dropwise addition of 0.05M sodium hydroxide aqueous solution. Theamount of carboxyl groups in the oxidized cellulose nanofibers wascalculated using the following equation based on the volume (V) ofsodium hydroxide consumed during the neutralization stage of the weakacid where changes in electrical conductivity are moderate:

Carboxyl group amount (mmol/g)={V(mL)×0.05}/mass(g) of pulp sample

<Number-Average Fiber Diameter>

The metal-containing oxidized cellulose nanofiber dispersion was dilutedto prepare a dispersion containing 0.0001% by mass of metal-containingoxidized cellulose nanofibers. The dispersion was dropped on mica anddried to form an observation sample. The sample was then observed usingan atomic force microscope (Dimension Fast Scan AFM, Bruker; tappingmode), and in an image in which metal-containing oxidized cellulosenanofibers can be confirmed, 5 or more metal-containing oxidizedcellulose nanofibers were measured for their fiber diameter and anaverage value was calculated.

<Number-Average Fiber Length>

The metal-containing oxidized cellulose nanofiber dispersion was dilutedto prepare a dispersion containing 0.0001% by mass of metal-containingoxidized cellulose nanofibers. The resulting dispersion was dropped onmica and dried to form an observation sample. The sample was thenobserved using an atomic force microscope (Dimension Fast Scan AFM,Bruker; tapping mode), and in an image in which metal-containingoxidized cellulose nanofibers can be confirmed, 5 or moremetal-containing oxidized cellulose nanofibers were measured for theirfiber length and an average value was calculated.

<Degree of Polymerization>

The prepared metal-containing oxidized cellulose nanofibers were reducedwith sodium borohydride to reduce remaining aldehyde groups in themolecules to their alcohols. Thereafter, the metal-containing oxidizedcellulose nanofibers subjected to the reduction treatment were dissolvedin a 0.5M copper ethylenediamine solution and the degree ofpolymerization was determined by the viscosity method. Specifically, thedegree of polymerization was determined in accordance with “Isogai, A.,Mutoh, N., Onabe, F., Usuda, M., “Viscosity measurements ofcellulose/SO²⁻amine-dimethylsulfoxide solution”, Sen'i Gakkaishi, 45,299-306 (1989).”

The reduction treatment using sodium borohydride was carried out inorder to prevent molecular weight reductions due to the beta eliminationreaction that occurs in the process of dissolution into the copperethylenediamine solution when aldehyde groups remained.

<Metal Amount>

Metals in the metal-containing oxidized cellulose nanofibers werequalified and quantified by ICP-AES. SPS5100 (SII NanoTechnology) wasused for the measurement. In addition, the amount of each ion wasquantified by ion chromatography. For measurement, DX-500 (DIONEX) wasused.

From the measurement results, the amounts of metals forming salts withcarboxyl groups of the oxidized cellulose nanofibers were determined.

Example 1 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

1 g in dry weight of coniferous bleached kraft pulp, 5 mmol of sodiumhypochlorite and 0.1 g (1 mmol) of sodium bromide as a co-oxidizingagent, and 0.016 g (1 mmol) of TEMPO as an oxidation catalyst weredispersed in 100 mL of water and stirred gently for 4 hours at roomtemperature to oxidize the coniferous bleached kraft pulp byTEMPO-catalyzed oxidation. The resulting oxidized pulp was washed withdistilled water to afford TEMPO-oxidized pulp (oxidized cellulose). Theamount of carboxyl groups of the TEMPO-oxidized pulp was 1.4 mmol/g.

Distilled water was then added to the never-dried TEMPO-oxidized pulp toprepare a dispersion having a solid content concentration of 0.1%. Thedispersion was subjected to defibration treatment for 2 minutes at7.5×1,000 rpm using a homogenizer (Physcotron, Microtec Co., Ltd.) andfor 4 minutes using an ultrasonic homogenizer (Ultrasonic Generator,Nissei Corporation; V-LEVEL: 4, TIP: 26D) while ice-cooling thesurroundings of the container. In this way, an aqueous dispersioncontaining carboxylated cellulose nanofibers as oxidized cellulosenanofibers was obtained. Thereafter, centrifugation (×12,000 g (120×100rpm/g), 10 min, 12° C.) was performed using a centrifugal separator(M201-1VD, angle rotor: 50E-8AL, SAKUMA) to remove non-defibratedcomponents from the aqueous dispersion of carboxylated cellulosenanofibers to afford 0.1% clear carboxylated cellulose nanofiber aqueousdispersion 1. The carboxylated cellulose nanofibers contained sodium(first metal) in salt form which was derived from the co-oxidizingagent.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

50 g of carboxylated cellulose nanofiber aqueous dispersion 1 wasstirred, 18 g of a 0.1% copper (II) acetate aqueous solution was addedas an aqueous solution of a second metal salt, and stirring wascontinued at room temperature for 3 hours (metal substitution step).

The carboxylated cellulose nanofibers gelled by the addition of thecopper (II) acetate aqueous solution were then recovered bycentrifugation (×12,000 g (120×100 rpm/g), 10 min, 12° C.) using acentrifugal separator (M201-1VD, angle rotor: 50E-8AL, SAKUMA), and therecovered carboxylated cellulose nanofibers were washed with 0.1% copper(II) acetate aqueous solution and then with a large quantity ofdistilled water (washing step).

50 ml of distilled water was then added and the dispersion was subjectedto ultrasonic treatment for 2 minutes using an ultrasonic homogenizer(Ultrasonic Generator, Nissei Corporation; V-LEVEL: 4, TIP: 26D) whileice-cooling the surroundings of the container to dispersemetal-substituted carboxylated cellulose nanofibers. Centrifugation(×12,000 g (120×100 rpm/g), 10 min, 12° C.) was performed using acentrifugal separator (M201-1VD, angle rotor: 50E-8AL, SAKUMA) to removenon-defibrated components to afford a 0.1% clear aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers (dispersing step).

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water. The relationship between birefringence anddispersibility is disclosed in WO2009/069641, for example.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.13 nm and a number-average fiberlength of 550 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 600.

Further, as a result of ICP-AES measurement, it was found that copper(Cu) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-half that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass. These results suggestthat sodium ions present in the carboxylated cellulose nanofibers of themetal-containing carboxylated cellulose nanofibers are substituted bycopper ions and one copper ion is bound per two carboxyl groups.

Example 2 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 1.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 1 except that 19 g of 0.1% cobalt(II) acetate aqueous solution was used instead of 18 g of 0.1% copper(II) acetate aqueous solution in the metal substitution step, and that0.1% cobalt (II) acetate aqueous solution was used instead of 0.1%copper (II) acetate aqueous solution in the washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.15 nm and a number-average fiberlength of 560 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 650.

Further, as a result of ICP-AES measurement, it was found that cobalt(Co) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-half that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass. These results suggestthat sodium ions present in the carboxylated cellulose nanofibers of themetal-containing carboxylated cellulose nanofibers are substituted bycobalt ions and one cobalt ion is bound per two carboxyl groups.

Example 3 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 1.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 1 except that 26 g of 0.1%aluminum (III) chloride hexahydrate aqueous solution was used instead of18 g of 0.1% copper (II) acetate aqueous solution in the metalsubstitution step, and that 0.1% aluminum (III) chloride hexahydrateaqueous solution was used instead of 0.1% copper (II) acetate aqueoussolution in the washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.14 nm and a number-average fiberlength of 500 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 550.

Further, as a result of ICP-AES measurement, it was found that aluminum(Al) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-third that in moles of the carboxyl groupsof the carboxylated cellulose nanofibers, and the amount of sodium wasnot greater than 1 ppm by mass. Further, as a result of quantitation ofthe ion amount by ion chromatography, it was found that the amount ofchloride ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by aluminum ions and one aluminum ion is bound per threecarboxyl groups.

Example 4 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 1.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

To 100 mL of carboxylated cellulose nanofiber aqueous dispersion 1 wasadded 1 mL of 1M hydrochloric acid under stirring to adjust the pH to 1.Stirring was continued for 60 minutes (hydrogen substitution step).

Thereafter, the carboxylated cellulose nanofibers gelled by the additionof hydrochloric acid were recovered by centrifugation (×12,000 g(120×100 rpm/g), 10 min, 12° C.) using a centrifugal separator(M201-1VD, angle rotor: 50E-8AL, SAKUMA), and the recovered carboxylatedcellulose nanofibers were washed with 1M hydrochloric acid and then witha large quantity of distilled water (first washing step).

Next, 100 mL of distilled water was added to afford 0.1%hydrogen-substituted carboxylated cellulose nanofiber aqueous dispersion1 in which hydrogen-substituted carboxylated cellulose nanofibers aredispersed (first dispersing step). 90% or more of the carboxyl groupspresent on the surface of the hydrogen-substituted carboxylatedcellulose nanofibers were substituted by carboxylic acid form asmeasured by FT-IR (FT/IR-6100, JASCO Corporation) in accordance withBiomacromolecules, 2011, vol. 12, pp. 518-522.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

50 g of 0.1% hydrogen-substituted carboxylated cellulose nanofiberaqueous dispersion 1 was stirred, 18 g of a 0.1% copper (II) acetateaqueous solution was added as an aqueous solution of a second metalsalt, and stirring was continued at room temperature for 3 hours (metalsubstitution step).

The carboxylated cellulose nanofibers gelled by the addition of thecopper (II) acetate aqueous solution were then recovered bycentrifugation (×2,000 g (120×100 rpm/g), 10 min, 12° C.) using acentrifugal separator (M201-1VD, angle rotor: 50E-8AL, SAKUMA), and therecovered carboxylated cellulose nanofibers were washed with 0.1% copper(II) acetate aqueous solution and then with a large quantity ofdistilled water (second washing step).

50 ml of distilled water was then added and the dispersion was subjectedto ultrasonic treatment for 2 minutes using an ultrasonic homogenizer(Ultrasonic Generator, Nissei Corporation; V-LEVEL: 4, TIP: 26D) whileice-cooling the surroundings of the container to dispersemetal-substituted carboxylated cellulose nanofibers. Centrifugation(×12,000 g (120×100 rpm/g), 10 min, 12° C.) was performed using acentrifugal separator (M201-1VD, angle rotor: 50E-8AL, SAKUMA) to removenon-defibrated components to afford a 0.1% clear aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers (second dispersingstep).

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.13 nm and a number-average fiberlength of 530 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 580.

Further, as a result of ICP-AES measurement, it was found that copper(Cu) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-half that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by copper ions and one copper ion is bound per two carboxylgroups.

Example 5 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 4.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

0.1% hydrogen-substituted carboxylated cellulose nanofiber aqueousdispersion 1 was prepared as in Example 4.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 4 except that 19.5 g of 0.1% zinc(II) acetate aqueous solution was used instead of 18 g of 0.1% copper(II) acetate aqueous solution in the metal substitution step, and that0.1% zinc (II) acetate aqueous solution was used instead of 0.1% copper(II) acetate aqueous solution in the second washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.15 nm and a number-average fiberlength of 520 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 560.

Further, as a result of ICP-AES measurement, it was found that zinc (Zn)was present in the metal-containing carboxylated cellulose nanofibers atan amount one-half that in moles of the carboxyl groups of thecarboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by zinc ions and one zinc ion is bound per two carboxylgroups.

Example 6 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 4.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

0.1% hydrogen-substituted carboxylated cellulose nanofiber aqueousdispersion 1 was prepared as in Example 4.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 4 except that 19 g of 0.1% cobalt(II) acetate aqueous solution was used instead of 18 g of 0.1% copper(II) acetate aqueous solution in the metal substitution step, and that0.1% cobalt (II) acetate aqueous solution was used instead of 0.1%copper (II) acetate aqueous solution in the second washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.15 nm and a number-average fiberlength of 550 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 600.

Further, as a result of ICP-AES measurement, it was found that cobalt(Co) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-half that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by cobalt ions and one cobalt ion is bound per two carboxylgroups.

Example 7 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 4.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

0.1% hydrogen-substituted carboxylated cellulose nanofiber aqueousdispersion 1 was prepared as in Example 4.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 4 except that 19 g of 0.1% calcium(II) acetate monohydrate aqueous solution was used instead of 18 g of0.1% copper (II) acetate aqueous solution in the metal substitutionstep, and that 0.1% calcium (II) acetate monohydrate aqueous solutionwas used instead of 0.1% copper (II) acetate aqueous solution in thesecond washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.14 nm and a number-average fiberlength of 550 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 600.

Further, as a result of ICP-AES measurement, it was found that calcium(Ca) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-half that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by calcium ions and one calcium ion is bound per twocarboxyl groups.

Example 8 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 4.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

0.1% hydrogen-substituted carboxylated cellulose nanofiber aqueousdispersion 1 was prepared as in Example 4.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 4 except that 18 g of 0.1% silver(I) acetate aqueous solution was used instead of 18 g of 0.1% copper(II) acetate aqueous solution in the metal substitution step, and that0.1% silver (I) acetate aqueous solution was used instead of 0.1% copper(II) acetate aqueous solution in the second washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side from the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.13 nm and a number-average fiberlength of 540 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 590.

Further, as a result of ICP-AES measurement, it was found that silver(Ag) was present in the metal-containing carboxylated cellulosenanofibers at an amount equal to that in moles of the carboxyl groups ofthe carboxylated cellulose nanofibers, and the amount of sodium was notgreater than 1 ppm by mass. Further, as a result of quantitation of theion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated nanofibers aresubstituted by silver ions and one silver ion is bound per one carboxylgroup.

Example 9 <Preparation of Dispersion of Oxidized Cellulose Nanofibers>

0.1% carboxylated cellulose nanofiber aqueous dispersion 1 was preparedas in Example 4.

<Preparation of Dispersion of Hydrogen-Substituted Oxidized CelluloseNanofibers>

0.1% hydrogen-substituted carboxylated cellulose nanofiber aqueousdispersion 1 was prepared as in Example 4.

<Preparation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

A 0.1% aqueous dispersion of metal-containing carboxylated cellulosenanofibers was prepared as in Example 4 except that 26 g of 0.1%aluminum (III) chloride hexahydrate aqueous solution was used instead of18 g of 0.1% copper (II) acetate aqueous solution in the metalsubstitution step, and that 0.1% aluminum (III) chloride hexahydrateaqueous solution was used instead of 0.1% copper (II) acetate aqueoussolution in the second washing step.

<Evaluation of Dispersion of Metal-Containing Oxidized CelluloseNanofibers>

Birefringence was observed when the resulting aqueous dispersion ofmetal-containing carboxylated cellulose nanofibers was placed betweentwo polarizing plates arranged in crossed Nicols and the aqueousdispersion was allowed to move in between the polarizing plates whiledirecting light from the opposite side to the viewer. This confirmedthat the metal-containing carboxylated cellulose nanofibers were welldispersed in water.

The metal-containing carboxylated cellulose nanofibers had anumber-average fiber diameter of 3.15 nm and a number-average fiberlength of 490 nm, confirming that the metal-containing carboxylatedcellulose nanofibers were dispersed in water at the microfibril level.The metal-containing carboxylated cellulose nanofibers had an averagedegree of polymerization of 530.

Further, as a result of ICP-AES measurement, it was found that aluminum(Al) was present in the metal-containing carboxylated cellulosenanofibers at an amount one-third that in moles of the carboxyl groupsof the carboxylated cellulose nanofibers, and the amount of sodium wasnot greater than 1 ppm by mass. Further, as a result of quantitation ofthe ion amount by ion chromatography, it was found that the amount ofacetate ions was not greater than 0.5 ppm by mass and the amount ofchlorine ions was not greater than 0.1 ppm by mass. These resultssuggest that sodium ions present in the carboxylated cellulosenanofibers of the metal-containing carboxylated cellulose nanofibers aresubstituted by aluminum ions and one aluminum ion is bound per threecarboxyl groups.

It can be seen from Examples 1 to 9 that the disclosed production methodcan provide a dispersion of metal-containing oxidized cellulosenanofibers with superior dispersibility, which is applicable to varioususes.

INDUSTRIAL APPLICABILITY

According to the present disclosure, it is possible to provide adispersion of metal-containing oxidized cellulose nanofibers withsuperior dispersibility, which is applicable to various uses.

1. A metal-containing oxidized cellulose nanofiber dispersioncomprising: a dispersion medium; and metal-containing oxidized cellulosenanofibers containing a metal other than sodium in salt form, whereinthe metal-containing oxidized cellulose nanofibers have a number-averagefiber diameter of 100 nm or less.
 2. The metal-containing oxidizedcellulose nanofiber dispersion of claim 1, wherein the metal-containingoxidized cellulose nanofibers are metal-containing carboxylatedcellulose nanofibers.
 3. The metal-containing oxidized cellulosenanofiber dispersion of claim 1, wherein the metal-containing oxidizedcellulose nanofibers have a number-average fiber length of 50 nm to2,000 nm.
 4. The metal-containing oxidized cellulose nanofiberdispersion of claim 1, wherein the metal-containing oxidized cellulosenanofibers have an average degree of polymerization of 100 to 2,000. 5.The metal-containing oxidized cellulose nanofiber dispersion of claim 1,wherein the metal other than sodium is at least one metal selected fromthe group consisting of metals of Group 2 to Group 14 in Period 3 toPeriod 6 of the long periodic table.
 6. The metal-containing oxidizedcellulose nanofiber dispersion of claim 1, wherein the metal other thansodium is at least one metal selected from the group consisting ofmagnesium, aluminum, calcium, titanium, chromium, manganese, iron,cobalt, nickel, copper, zinc, silver, tin, barium, and lead.
 7. Themetal-containing oxidized cellulose nanofiber dispersion of claim 1,wherein the metal other than sodium is at least one metal selected fromthe group consisting of aluminum, calcium, iron, cobalt, copper, zinc,and silver.
 8. The metal-containing oxidized cellulose nanofiberdispersion of claim 1, wherein the dispersion medium is water.
 9. Amethod of producing a dispersion of metal-containing oxidized cellulosenanofibers having a number-average fiber diameter of 100 nm or less, themethod comprising: contacting oxidized cellulose nanofibers containing afirst metal in salt form, dispersed in a solvent, with a salt of asecond metal other than the first metal to provide metal-containingoxidized cellulose nanofibers containing the second metal in salt form.10. A method of producing a dispersion of metal-containing oxidizedcellulose nanofibers having a number-average fiber diameter of 100 nm orless, the method comprising: contacting oxidized cellulose nanofiberscontaining a first metal in salt form, dispersed in a solvent, with astrong acid to substitute ions of the first metal contained in salt formby hydrogen atoms; and contacting the oxidized cellulose nanofibers inwhich the ions of the first metal have been substituted by hydrogenatoms, dispersed in a solvent, with a salt of a second metal other thanthe first metal to provide metal-containing oxidized cellulosenanofibers containing the second metal in salt form.
 11. The method ofproducing a dispersion of metal-containing oxidized cellulose nanofibersof claim 10, wherein the oxidized cellulose nanofibers are carboxylatedcellulose nanofibers.
 12. The method of producing a dispersion ofmetal-containing oxidized cellulose nanofibers of claim 10, wherein themetal-containing oxidized cellulose nanofibers have a number-averagefiber length of 50 nm to 2,000 nm.
 13. The method of producing adispersion of metal-containing oxidized cellulose nanofibers of claim10, wherein the metal-containing oxidized cellulose nanofibers have anaverage degree of polymerization of 100 to 2,000.
 14. The method ofproducing a dispersion of metal-containing oxidized cellulose nanofibersof claim 10, wherein the first metal is sodium, and the second metal isat least one metal selected from the group consisting of metals of Group2 to Group 14 in Period 3 to Period 6 of the long periodic table. 15.The method of producing a dispersion of metal-containing oxidizedcellulose nanofibers of claim 10, wherein the first metal is sodium, andthe second metal is at least one metal selected from the groupconsisting of magnesium, aluminum, calcium, titanium, chromium,manganese, iron, cobalt, nickel, copper, zinc, silver, tin, barium, andlead.
 16. The method of producing a dispersion of metal-containingoxidized cellulose nanofibers of claim 10, wherein the first metal issodium, and the second metal is at least one metal selected from thegroup consisting of aluminum, calcium, iron, cobalt, copper, zinc, andsilver.
 17. The method of producing a dispersion of metal-containingoxidized cellulose nanofibers of claim 10, wherein the solvent is water.